Electronic oven with ferrite rf rejection filters

ABSTRACT

There is disclosed an electronic oven comprising a generator of microwave energy of a predetermined frequency coupled to a cooking cavity and to a source of DC and 60 cycle operating potentials by a coupling structure, the coupling structure including an RF rejection filter comprising coaxial inner and outer conductors insulated from each other, the inner conductor having a cylindrical ferrite member and a cylindrical metal slug therein of such diameter that the filter operates as a lossy transmission line, terminated in a capacitance for highly attenuating the predetermined frequency and all harmonics thereof up to the seventh harmonic.

United States Patent [72] Inventor Louis H. Fitzmayer 3,329,911 7/1967Schlicke et a1.... 333/79 Loulavllle, 3,456,215 7/1969 Denes 333/79 [21]Appl. No. 877,989 3,462,715 8/1969 Schor 3313/79 [22] Filed Nov. 19,19693,511,958 /1970 Staats 219/1055 [45] Patented Dec. 7, 1971 PrimaryExammer-J. V. Truhe [73] Asslgnee General Electric company AssistantExaminer-Hugh D. .laeger Attorneys-- Prangley, Clayton, Mullin, Dithmar& Vogel, [54] ELECTRONIC OVEN WITH FERRITE RF Oscar B. Waddell, Frank L.Neuhauser, Richard L. Caslin REJECTION FILTERS and Harry F. Manbeck, J1.8 Claims, 7 Drawing Figs.

[52] US. Cl 219/ 10.55, ABSTRACT: There is disclosed an electronic ovencomprising 333/73 C a generator of microwave energy of a predeterminedfrequen- [51] Int. Cl 1105b 9/06 cy coupled to a cooking cavity and teasource of DC and 60 [50] Field of Search 219/1055; cycle operatingpotentials by a coupling structure, the 3333/73 C, 79 coupling structureincluding an RF rejection filter comprising coaxial inner and outerconductors insulated from each other, [56] Cited the inner conductorhaving a cylindrical ferrite member and a UNITED STATES PATENTScylindrical metal slug therein of such diameter that the filter3,536,878 10/1970 Fitzmayer et a1. 219/1055 Operates as a lossytransmission line, terminated in 3 2 700 3 9 5 5 Devot u 333 73capacitance for highly attenuating the predetermined frequen- 2,877,4333/1959 Devotmf. 333/73 c Cy and harmonics thereof p to the Seventhharmonic 27 Y L2 I r L.- 4/ 22 7 c 1. 46 9 36 a/ g 2/ 3e l|:| l

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INVENTOR LOU/S H. F/ TZMA YER ATT).

ELECTRONIC OVEN WITH FERRlTE RF REJECTION FILTERS This invention isconcerned with an improved microwave coupling structure and an improvedRF rejection filter therefore forming a part of an electronic oven.

More particularly, this invention concerns an improved microwaverejection filter for microwave coupling structure in an electronic oven.

It is a general object of this invention to provide an RF rejectionfilter of simple and economical construction and which affords highattenuationof the frequency of operation of the microwave generator inan electronic oven and also provides high attenuation of the harmonicsof this frequency of operation up to and including the seventh harmonic.

It is a particular object of this invention to provide in an electronicheating apparatus adapted for use with a source of DC operatingpotential having at least one terminal and with a' source of lowfrequency AC energy having at least one terminal, structure defining acooking cavity, a generator for generating electromagnetic wave energyof a predetermined ultrahigh frequency and having a pair of outputterminals, a transmission line coupling the generator to the cookingcavity, a hollow outer conductor'coupled to one output terminal of thegenerator and extending outwardly therefrom and terminating in an outerend, an inner conductor disposed in the outer conductor and coupled tothe other output terminal of the generator and extending outwardlytherefrom andter-V minating in an outer end adjacent to the outer end ofthe outer conductor, an insulating'sleeve disposed within the outerconductor in surrounding relationship with the inner conductor, meanscoupling the DC source terminal and the low frequency AC source terminalto the outer end of the inner conductor, a ferrite filter elementdisposed in theinner conductor intermediate the outer end thereof andthe generator output terminals and being constructed and arranged toprovide, hightransmission losses at selected harmonics of thepredetermined frequency, and a conductive filter element disposed in theinner conductor adjacent to the outer end thereof and connected to theferrite filterelement and being constructed and arranged to cooperatewith the insulating sleeve and the outer conductor for providing alow-impedance bypass path between the inner and outer conductors at thepredetermined frequency, whereby propagation of ultrahigh frequencyenergy at the predeterminedfrequency and selected harmonics thereof fromthe generator to the outer ends of the conductors is highly attenuatedwithoutinterfering with the transmission of the DC operating potentialsand the low frequency AC energy from the sources thereof to thegenerator.

In connection with the foregoing object, it is another object of thisinvention to provide an electronic-heating apparatus of the type setforth, wherein the generator has a low-frequency AC input terminal andthe source of low-frequency AC energy has a pair of terminals, andfurther including a second hollow outer conductor coupled, to the oneoutput terminal of the generator and disposed adjacent to thelow-frequency AC input terminal and extending outwardly therefrom andter' minating in an outer end, a second inner conductor disposed in thesecond outer conductor and coupled to the lowfreq'uency AC inputterminal of the generator and extending outwardly therefrom andterminating in an outer end adjacent to the outer end of the secondinner conductor, a second insulating sleeve disposed within the secondouter conductor in surrounding relationship with the second innerconductor, means coupling the other of the low-frequency AC sourceterminals to the outer end of the second inner conductor a secondferrite filter element disposed in the second inner conductorintermediate the outer end thereof and the generator low-frequency ACinput terminal and being constructed and arranged to providehigh-transmission losses at selected harmonics of the predeterminedfrequency, and a second conductive filter element disposed in the secondinner conductor adjacent to the outer end thereof and connected to thesecond ferrite filter element and being constructed and arranged tocooperate with the second insulating sleeve and the second outerconductor for providing a low-impedance bypass path between the secondinner and other conductors for the predetermined frequency, wherebypropagation of ultrahigh frequency energy at the predetermined frequencyand harmonies thereof from the generator to the outer ends of the firstand second conductors is highly attenuated without interferring with thetransmission of the DC operating potentials and the low frequency ACenergy from the sources thereof to the generator. I

It is a further object of this invention to provide a coupling structurefor interconnecting a generator for generating RF energy of apredetermined ultrahigh RF frequency and a source of DC operatingpotentials having at least one terminal and a source of low-frequency ACenergy having at least one terminal, wherein the generator has anannular outer RF terminal and an inner RF terminal, the couplingstructure comprising a hallow outer conductor coupled to the outer RFterminal of the generator and extending outwardly therefrom andterminating in an outer end, an inner conductor disposed in. the outerconductor and coupled to the inner RF terminal of the generator andextending outwardly therefrom and terminating in an outer end adjacentto the outer end of the outer conductor, an insulating sleeve disposedwithin the outer conductor in surrounding relationship with the innerconductor, a pair of RF output terminals respectively coupled to theouter conductor and the inner conductor intermediate the generator RFterminals and the outer ends, means coupling the DC source terminal andthe low-frequency AC source terminal to the outer end of the innerconductor, a ferrite filter element disposed in the inner conductorintermediate the outer end thereof and the generator output terminalsand being constructed and arranged to provide high-transmission lossesat selected harmonics of the predetermined frequency, and a conductivefilter element disposed in the inner conductor adjacent to the outer endthereof and connected to the ferrite filter element and beingconstructed and arranged to cooperate with the insulating sleeve and theouter conductor for providing a low-impedance bypass path between theinner and outer conductors at the predetennined frequency, wherebypropagation of ultrahigh frequency energy at the predetermined frequencyand selected harmonics thereof from the generator to the outer ends ofthe conductors is highly attenuated without interfering with thetransmission of the DC operating potentials and the low-frequency ACenergy from the sources thereof to the generator.

in connection with the foregoing object, it is a further object of thisinvention to provide a coupling structure of the type set forth whereinthe source of low frequency AC energy has a pair of terminals and thegenerator has a low-frequency AC terminal, and further including asecond hollow outer conductor coupled to the one output terminal of thegenerator and disposed adjacent to the low-frequency AC input terminaland extending outwardly therefrom and terminating in an outer end, asecond inner conductor disposed in the second outer conductor andcoupled to the low-frequency AC input terminal of the generator andextending outwardly therefrom and terminating in an outer end adjacentto the outer end of the second inner conductor, a second insulatingsleeve disposed within the second outer conductor in surroundingrelationship with the second inner conductor, means coupling the otherof the low-frequency AC source terminals to the outer end of the secondinner conductor, a second ferrite filter element disposed in the secondinner conductor intermediate the outer end thereof and the generatorlow-frequency AC input terminal and being constructed and arrange toprovide hightransmission looses at selected harmonics of thepredetermined frequency, and a second conductive filter element disposedin the second inner conductor adjacent to the outer end thereof andconnected to the second ferrite filter element and being constructed andarranged to cooperate with the second insulating sleeve and the secondouter conductor for providing a low-impedance bypass path between thesecond inner and outer conductors at the predetennined frequency,

whereby propagation of ultrahigh frequency energy at the predeterminedfrequency and selected harmonics thereof from the generator to the outerends of the first and second conductors in highly attenuated withoutinterfering with the transmission of the DC operating potentials and thelowfrequency AC energy from the sources thereof to the generator.

Yet another object of this invention is to provide a RF rejection filterfor highly attenuating. a predetermined ultrahigh frequency and selectedharmonics thereof, the filter assembly comprising a coaxial transmissionline section including a hollow outer conductor and an inner conductordisposed in the outer conductor, a ferrite filter element disposed inthe inner conductor and being constructed and arranged to providehigh-transmission losses at selected harmonics of the predeterminedfrequency, a conductive filter element disposed in the inner conductorand connected to the ferrite filter element and being constructed andarranged to provide a low-impedance bypass path between the inner andouter conductors at the predetermined frequency, and an insulatingsleeve disposed between the filter elements and the outer conductor, thefilter elements cooperating with each other and with the inner and outerconductors to form a filter for highly attenuating the predeterminedfrequency and selected harmonics thereof. The invention, both as to itsorganization and method of operation, together with further objects andadvantages thereof will best be understood by reference to the followingspecification taken together with the accompanying drawings, in which:

FIG. 1 is a side elevational viewof an electronic-heating apparatus madein accordance with and embodying the principles of the present inventionwith the sidewall partly broken away to show the internal components;

FIG. 2 is an enlarged fragmentary view in vertical section of a crossedfield discharge device forming a part of the generator for the heatingapparatus of FIG. 1;

FIG. 3 is a further enlarged fragmentary sectional the of the left-handend of the microwave coupling structure shown in FIG. 2;

FIG. 4 is a sectional view taken along he line 44 in FIG. 3; FIG. 5 is asectional view taken along the line 5-5 in FIG.

FIG. 6 is a reduced top plan view of the crossed field discharge deviceand microwave coupling structure shown in FIG. 2; and

FIG. 7 is a bottom plan view of the crossed field discharge device shownin FIG. 2 with the RF rejection filter shown in cross section.

Referring now to FIG. 1 of the drawings, the electronicheating apparatus10 there illustrated, and embodying the features of the presentinvention, is in the form of a combination electric and electronic rangethat is especially designed for home use. More particularly, the range10 comprises an upstanding substantially boxlike casing 11 formed ofsteel and including a pair of sidewalls 12, a rear wall 13 having aremova' ble closure member or panel disposed therein, and a top wall 14and a bottom wall 16, the removable panel 15 being removably held inplace by a plurality of screws 27. The casing 11 houses in the uppercentral portion thereof a metal liner defining a heating or cookingcavity 21 therein, the metal liner 20 preferably being formed of steel,and essentially comprising a boxlike structure provided with a top wall22, a bottom wall 23, a rear wall 24, and a pair of opposed sidewallswhereby the liner 20 is provided with an upstanding front opening intothe heating cavity 21 defined therein. Further, the casing 11 isprovided with a front door 28 arranged in a front opening formed thereinand cooperating with the front opening provided in the liner 20, thefront door 28 being mounted adjacent to the lower end thereof uponassociated hinge structure 29; whereby the front door 28 is movablebetween a substantially vertical closed position and a substantiallyhorizontal open position with respect to the front opening provided inthe liner 20.

An electric heating unit 36 is arranged in the upper portion of theheating cavity 21 adjacent to and below the top wall 22, and an electricheating unit 37 is arranged in the lower portion of the heating cavity21 adjacent to and above the bottom wall 23; which electric heatingunits 36 and 37 are utilized in the carrying out of conventional bakingand broiling cooking operations in the heating cavity 21, as explainedmore fully hereinafter. Also, a temperature sensing bulb 38 is arrangedin a pocket provided in one of the sidewalls 25; which temperaturesensing bulb 38 forms a part of an oven switch and temperaturecontroller and is utilized in carrying out the previously mentionedconventional baking and broiling operations in the heating cavity 21.The sidewalls 25 of the liner 20 further carry thereon a plurality ofshelf supports 26 for the supporting of shelves (not shown) that in turnsupport items to be cooked within the heating cavity 21. There also isprovided below the front door 28 a lower front panel 19 that closes afront opening in the lower portion of the casing 11, the bottom wall 16further being provided with a reticulated member or screen 17 and thelower portion of the rear wall 13 being provided with a reticulatedmember or screen 18, the screens 17 and 18 permitting the passage of airthrough the lower portion of the casing 11 to cool the electronicapparatus therein as will be described more fully hereinafter.

Disposed below the liner 20 and spaced therefrom is a generallyhorizontally arranged lower baffle 30 extending laterally across thecasing 11 between the sidewalls 12 thereof and extending rearwardly fromthe front of the casing 11 to a point spaced forwardly of the rear wall13, the baffle 30 in cooperation with the casing 11 defining a bottommachinery compartment 35 in the lower portion of the casing 11, thelower front panel 19 being removable to provide access to the bottommachinery compartment 35.

Mounted on the underside of the baffle 30 is an electric motor 31 havingan output drive shaft 32 including a reduced portion 33 extendingupwardly into the liner 20 and supporting thereon a turntable 34 formedof perforated sheet metal and arranged in the lower portion of theheating cavity 21. The turntable 34 is mounted for rotation and uponoperation of the motor 31 is adapted to support food to be heated orcooked in the electronic operation that is carried out in the heatingcavity 21, as explained more fully hereinafter. The motor 31 has anassociated gear train (not shown) that reduces the speed of the shaft 33to approximately 6 rpm.

A rear bafile 40 is provided to the rear of the liner 20 and extendsacross the casing 1 1 between the sidewalls 12 thereof, the baffle 40including a main wall 41 carrying centrally thereof a rearwardly offsetwall 42 that is in general horizontal alignment with the rear of theliner 20 and spaced rearwardly therefrom. Disposed in the lower portionof the main wall 41 is an opening around which is disposed a flange 43connecting with an air duct 44 that communicates with the screen 18 inthe rear wall 13 of the casing 11. It further will be noted that thebottom baffle 30 carries on the rear thereof an upwardly and rearwardlyextending baffle section 39 that extends toward the offset wall 42 butis spaced therefrom, the baffles 30 and 40 being formed of metal, suchas steel, whereby the spacing between the baffle section 39 and the rearbaffle 40 minimizes the conduction of heat therebetween during theoperation of the range 10 the rear baffle 40 cooperates with the casing11 to provide a rear machinery compartment 45, the rear machinerycompartment 45 being disposed behind the liner 20 and access theretobeing provided through the removable panel 15 that covers the opening inthe rear wall 13 described above.

There is arranged to the right (as viewed in FIG. 1) of the bottommachinery compartment 35 a generator, generally designated by thenumeral 50, for supplying ultrahigh frequency electromagnetic waveenergy for electronic cooking within the cooking cavity 21, thegenerator 50 including a crossed field electron discharge device,generally designated by the numeral 60, of the construction andarrangement disclosed in the copending application of James E. Staats,Ser. No.

559,267, filed June 21, 1966, now US. Pat. No. 3,458,755. Referring toFIGS. 1 and 2 of the drawings, it will be seen that the device 60 isdisposed within a boxlike structure of casing 61 that extends completelyabout the device 60 but is open on two opposed sides thereof, the sidesdisposed to the left and right in FIG. 2, the device 60 and theassociated parts therefor being mounted within and electricallyconnected to the casing 61. As will be explained more fully hereinafter,high-operating DC potentials are present on the casing 61, whereby it isdesirable electrically to isolate and shield the casing 61, and to thisend a pair of mounting rails 62 are provided in the bottom of themachinery compartment 35 for supporting the device 60 and the casing 61.The mounting rails 62 each carry a pair of insulators 63 adjacent to theopposite ends thereof, the easing 61 being mounted on the insulators 63by a pair of angle brackets 64, only one of which is shown, which spanthe mounting rails 62 adjacent to the ends thereof. Thus, the casing 61is insulated from the mounting rails 62 and from the casing 11.

As viewed in FIG. 1, the mounting rails 62 and the casing 61 mountedthereon are disposed to the rear of the bottom machinery compartment 35,and are disposed to the right within the bottom machinery compartment 35when the range is viewed from the front. In order to provide cooling airfor passage across the device 60, there has been provided an open-endedhousing 65 disposed to the left in FIG. 1 or in front of the casing 61and housing therein at least a part of a voltage doubler and rectifiercircuit 95 that supplies DC operating potentials to the device 60 andalso houses therein a fan 66 powered by a motor 67 within the housing65. An air duct 68 is provided about the fan 66 to direct air therefrominto a cooperating air duct 69 mounted on the casing 61, the ducts 68and 69 cooperating to facilitate direction of the airstream over thedevice 60. More specifically, the fan 66 operates to draw air throughthe screen 17 at the bottom of the machinery compartment 35, the airbeing formed into a stream by the housing 65, and passed over therectifier 95, the motor 55 and through the air ducts 68 and 69 into thecasing 61; the airstream within the casing 61 passes about the device 60and cooling fins 71 disposed thereon and passes therefrom and into theair duct 44 to be discharged through the screen 18 in the rear wall 13of the casing 11.

In accordance with the present invention, the airstream generated by thefan 66 if used to cool all of the various parts of the generator 50, andspecifically the crossed field discharge device 60 and the voltagedoubler and rectifier circuit 95 associated therewith. To this end thehousing 65 and the air ducts 68 and 69 and the air duct 44 have beenprovided so as to concentrate the airstream upon the parts noted, allwhile attempting to deflect the airstream away from the baffles 30 and40. The baffles 30 and 40 further protect the liner 20 and the cookingcavity 21 disposed therein from the airstream thus generated so as tomaintain more uniform cooking conditions within the cooking cavity 21and thus to improve the cooking therein.

Referring now particularly to FIG. 2 of the drawings, the generator 50there illustrated will be described in greater detail. An electrondischarge device 60 is contained within a substantially cylindricalmetal envelope 72 and includes anode and cathode structure (not shown).Surrounding the envelope 72 and connected thereto is a plurality ofcooling fins 71 for dissipating heat from the device 60 as explainedabove. In order to establish a unidirectional magneticfield within thedevice 60, there is provided a composite magnetic field winding 70(a)and 70(b) disposed at the upper and lower ends of the device 60 andconnected in series relation by a conductor 73. A DC operating potentialfrom the voltage doubler and rectifier 95 is applied to the winding 70aby a conductor 74, and from the winding 70b to the device 60 by aconductor 75 which is connected to one of the cooling fins 71 as at 750.Further details of the construction and operation of the device 60 aredisclosed in the previously mentioned Staats application Ser. No.559,267, now U.S. Pat. No. 3,458,755, the disclosure of which isincorporated herein by reference.

The generator 50 is arranged to be advantageously operated in connectionwith suitable control and power supply apparatus, details of theconstruction and operation of which are disclosed in the copending U.S.applications of James E. Staats, Ser. No. 656,977, filed June 12, 1967,now U.S. Pat. No. 3,421,1 l5; and Ser. No. 181,144, filed Mar. 20, 1962;and in the copending U.S. application of James E. Staats and Robert D.Ogbum, Ser. No. 676,584, filed Oct. 19, 1967, and now U.S. Pat. No.3,445,784, the disclosures of all of which applications are incorporatedherein by reference.

The crossed field discharge device 60 is provided with a magnet yoke 76at the upper end thereof which connects to the anode of the device 60(not shown) and forms an outer conductorand an output terminal for thedevice 60, the lower end of the magnetic yoke 76 being more particularlyconnected to the anode of the device 60 and the upper end extendingupwardly through the field winding 70a and being connected to a magnetflange 77. The lower end of the device 60 is likewise provided with amagnet yoke 78 having the upper end thereof connected to the anode ofthe device 60 and the other end extending downwardly through the fieldwinding 70b and being connected at the lower end to a magnet flange 79.The cathode of the device 60 (not shown) has connected thereto a stud101 forming a part of an upper coupling structure 100, the stud 101 andthe magnet yoke 78 forming a coaxial output connection for the device60.

The device 60 is operative to supply ultrahigh frequency energy of about915 me, with a power output at the output terminals in the general range50 to 800 watts. The device is arranged to supply the RF power forcooking and to this end a lower transmission line, generally designatedby the numeral 90, extends from the device 60 and is coupled to an uppertransmission line, generally designated by the numeral 80, which extendsto the cooking cavity 21, the transmission lines 80 and being of thecoaxial type including an inner conductor and an enclosing outerconductor electrically insulated therefrom. Both of the output terminalsof the device 60 are at a substantial DC voltage with respect to groundpotential, so that the output terminals are electrically insulated fromground potential, as well as from each other. One of the outputterminals is coupled by a capacitor (not shown) to the adjacent end ofthe outer outp'ut conductor, and the other output terminal is coupled bya capacitor (not shown) to the adjacent end of the inner outputconductor, the remote end of the inner conductor projecting as anantenna 81 into the oven cavity 21. The remote end of the outerconductor is electrically connected to the metal liner 20. Thus the RFpower produced by operation of the device 60 is radiated from theantenna 81 into the cooking cavity 21, so as to produce cooking effectsupon food arranged therein, all in a conventional manner.

The antenna 81 is essentially a rod connected at one end thereof to theinner conductor of the upper transmission line 80, the antenna 81extending into the cavity 21 a short distance below the top wall 22 andessentially midway between the sidewalls 25, as shown in FIG. 1. A topcoaxial transmission line section 82 of the upper transmission line 80is disposed adjacent to the antenna 81, the inner conductor of thetransmission line section 82 being connected to the antenna 81 and theouter conductor of the transmission line section 82 being connected tothe rear wall 24 of the cooking cavity liner 20. The rear end of thetransmission line section 82 is received in the forward arm of a tee 83in telescopic relationship therewith, the rear arm of the tee 83carrying on the outer end thereof a dust cover that serves to close thesame to prevent the entry of dust, dirt, and water or the like into theinterior of the tee 83. It further is pointed out that the forward armof the tee 83 extends through an opening 46 in the baffle wall 42,whereby the major portion of the tee 83 is disposed to the rear of thebaffle wall 42 and thus in the rear machinery compartment 45, wherebythe tee 83 is accessible for service and maintenance through theremovable panel 15. A rear coaxial section 84 for the upper transmissionline 80 is disposed in the rear machinery compartment 45, the upper endof the section 84 extending into the lower arm or leg of the tee 83 intelescopic relationship therewith. The inner conductors of thetransmission line sections 82 and 84 are connected together within thetee 83, preferably by means of a screw (not shown). The lower end of therear transmission line section 84 is received in the upper arm or leg ofa tee 85 in telescopic relationship therewith. The rear arm of the tee85 preferably carries at the outer end thereof a dust cover similar tothe dust cover on the tee 83, the forward arm of the tee 85 extendingthrough an opening 47 in the baffle wall 42 and receiving therein atelescopic relationship therewith the rearward end of tee lowertransmission line 90. Preferably, clamps 86 are provided around the armsof the tees 83 and 85 coupled to the various sections of thetransmission lines 80 and 90, these arms preferably being slotted (notshown) whereby tightening of the associated clamps 86 serves to tightenthe arms about the associated ends of the respective transmission lines80 and 90 to hold the parts in the assembled telescoping positions. Theinner conductor of the coaxial transmission line 90 is connected to theinner conductor of the rear section 84 of the upper transmission line 80within the tee 85, preferably by a screw (not shown).

Both the inner and outer conductors of the lower transmission line 90are divided into forward and rearward sections 91 and 92, respectively,separated from each other by insulating sleeves (not shown) to providethe capacitive coupling between the generator 50 and the oven liner 20to prevent the high DC potentials existing at the output tenninals ofthe generator 50 from existing on the oven liner 20 or the casing 11.Preferably, the forward end of the outer conductor of the rear sectiON92 is flared to facilitate reception thereinto of the rearward end ofthe forward section 91. The rear section of the inner conductor issimilarly flared (not shown). Further, a plurality of microwave filters(not shown) are provided in the lower transmission line 90 for highlyattenuating the harmonies of the frequency of operation of the device 60and preventing propagation of these harmonics to the oven cavity 21.Details of the construction and operation of the transmission lines 80and 90 are disclosed in the copending US. application of James. E StaatsSer. No. 788,204 filed Dec. 31, 1968, and now U.S. Pat. No. 3,511,958,the disclosure of which application is incorporated herein by reference.

An upper coupling structure, generally designated by the numeral 100, isprovided at the upper end of the device 60 for coupling the terminalsthereof to the lower transmission line 90 and to the voltage doubler andrectifier power supply 95. The forward end of the lower transmissionline 90 is coupled to the crossed field discharge device 60 by means ofa tee 110, the tee 110 including a body 111 having a pair of arms 112and 114 and a leg or arm 116 each provided with a cooperating internalseat or shoulder 113, 115 and 117, respectively. More particularly, theleg 116 is positioned downwardly and is telescopically received in theupper end of the magnet yoke-76 for coupling thereto, the tee 110forming a part of the upper coupling structure 100. Also forming a partof the upper coupling structure lis a bullet 102 receiving into thelower end thereof the stud 101 that is coupled to the cathode of thedevice 60, the bullet 102 having a shoulder 103 thereon that cooperateswith the shoulder 117 to hold in operative position an insulator 118that serves to center the bullet 102 with ,20 respect to the leg 116.The upper end of the bullet 102 carries an internally threaded eye 104through which is threadedly inserted a stud 105, one end of the stud 105being threadedly received in a complementarily threaded opening in aninner conductor 106 of the transmission line 90, thereby to support theforward end thereof, there also being provided therearound a supportinginsulator 107. The forward end of the outer conductor 108 of thetransmission line 90 is telescopically received within the leg 112 andis spaced a short distance away from the shoulder 113, thereby to clampthe insulator 107 in the operative position therebetween. Preferably theforward end of the outer conductor 108 is fixedly secured as bysoldering to the arm 112.

A decoupling structure, generally designated by the numeral 120, iscarried by the other arm 114 of the tee 110, and more particularlycomprises an inner conductor 121 having a rearward end connected to theforward end of the stud by means of a threaded connection therebetween,the inner conductor having a reduced forward portion 122. Fixedlymounted on the arm 114 and telescopically received therein is an outerconductor 124 within which is disposed an insulating sleeve 126. T1-1eouter conductor 124 and the sleeve 126 both extend outwardly beyond theouter end of the inner conductor 121. A cylindrical inner conductorhaving an outer diameter slightly less than the outer diameter of thereduced forward portion 122 of the inner conductor 121, is disposedforwardly of the inner conductor 121 along the longitudinal axisthereof, The inner conductor 125 is externally threaded adjacent to eachend thereof and is provided intermediate the ends thereof with aradially outwardly extending circular bearing flange or collar 129. Therearward end 127 of the inner conductor 125 is threadedly engaged with acomplementarily threaded opening 128 in the outer end of the reduceddiameter forward portion 122 of the inner conductor 121. Disposed withinthe insulating sleeve 126 is a cuplike inner conductor 130 having acylindrical side wall 131 surrounding the reduced portion 122 of theinner conductor 121 and having an outer end closed by an end wall 132which abuts the reduced portion 122 of the inner conductor 121 at theouter end thereof, the threaded portion 127 of the inner conductor 125passing through an opening in the wall 132.

Connected to the outer end of the outer conductor 124 in coaxialsurrounding relationship with the inner conductor 125 is a hollowcylindrical outer conductor 135 including a relatively large diameterinner section 136, a relatively small diameter outer section 138 and atapered section 137 interconnecting the inner and outer sections 136 and138. The outer end of the outer conductor 124 is disposed within thelarge diameter inner section 136 of the outer conductor 135 intelescoping relationship therewith and is preferably fixedly securedthereto as by clamping. The outer end of the insulating sleeve 126extends outwardly beyond the end of the outer conductor 124 and into thetapered section 137 of the outer conductor 135 and is shaped conformablytherewith. MOunted on the outer end of the inner conductor 125 is acylindrical ferrite member having an opening 141 extending axiallytherethrough for receiving therein the threaded outer end of the innerconductor 125 whereby the filter member 140 is mounted on the innerconductor 125 in surrounding relationship therewith. In its assembledposition, the inner end of the ferrite member 140 abuts against thebearing flange 129 on the inner conductor 125 and the outer end of theferrite member 140 extends into the reduced diameter portion 138 of theouter conductor 135. Disposed outwardly of the ferrite member 140 andadjacent thereto is a cylindrical metal slug having an internallythreaded opening 141 extending axially therethrough for receivingtherein and threadedly engaging with the threaded outer end of the innerconductor 125 for mounting the slug 150 thereon in surroundingrelationship therewith. In use, the metal slug 150 is threaded he innerconductor 125 until it abuts against the outer end of the ferrite member140 for holding the ferrite member 140 in its assembled position againstthe bearing flange 129. Disposed within the reduced diameter section 138of the outer conductor 135 in surrounding relationship with the ferritemember 140 and the metal slug 150 is an insulating sleeve 145. Aterminal screw 152 is preferably received in the outer end of theinternally threaded opening 151 of metal slug 150 fonning a terminal forconnecting a power supply lead 155 thereto.

Because of the effective filtration of harmonics of the operatingfrequency of the device 60 in the lower transmission line 90, it isnecessary that these harmonics, as well as the frequency of theoperation itself, be effectively filtered in the coupling structure 120described above to prevent their propagation to the Edison supplynetwork and to the voltage doubler and rectifier circuit 95. To thisend, the cuplike inner conductor 130 cooperates with the inner conductor121 to define a cavity having an effective electrical length equal toone quarter of the wavelength of the frequency of operation of thedevice 60 and being resonant thereat to provide a high-series impedancefor the transmission of this frequency along the conductors 121 and 124.The conductors 121 and124 cooperate with each other and with theinsulating sleeve 126 to provide a capacitive low-impedance bypass paththerebetween for the second harmonic of the frequency of the operationof the device 60. Similarly, the cuplike inner conductor 130 cooperateswith the outer conductor 124 to provide therebetween a low-impedancebypass path at the third harmonic of the frequency of operation of thedevice 60.

In effect, the inner conductor 121, the outer conductor 124 and thecuplike conductor 130 are so arranged and dimensioned that there isprovided along the inner conductor 121 capacitive loading on the secondharmonic of the frequency of operation of the device 60. There is alsoprovided at the inner end of the inner conductor 121 an open circuitreflection at the frequency of operation of the device 60 between theinner conductor 121 and the outer conductor 124; there is provided atthe inner end of the cuplike conductor 130 a short circuit reflection atthe third harmonic of the frequency of operation of the device 60between the conductors 130 and 124. Accordingly, the above described RFfilters provide high attenuation of the frequencies of operation of thedevice 60 and of the second and third harmonics thereof to minimize thepropagation of RF energy to the Edison supply network and to the voltagedoubler andthe rectifier circuit 95.

However, to insure that there is no appreciable RF leakage to the powersupply circuitry, additional rejection filters are preferably providedin the decoupling structure 120. To this end, the ferrite member 140 andthe metal slug 150 are provided in order to improve filtration of thefrequency of the operation of the device 60 and the harmonics thereofand further to extend the effective range of this filtration over a bandof frequencies including all harmonics up to the seventh harmonic. lnoperation, the ferrite member 140 acts like a section of lossytransmission line in the coaxial mode at frequencies above the frequencyof operation of the device 60, thereby producing high attenuation of theharmonics of the frequency of operation of the device 60, theattenuation increasing with frequency. Preferably, the outer diameter ofthe ferrite member 140 is such as to produce high attenuation of allhannonics up to the seventh harmonic of the frequency of operation. Themetal slug 150 preferably has an outer diameter substantially equal tothe outer diameter of the ferrite element 140 for ease of manufacture,this outer diameter being such that both the ferrite element 140 and themetal slug 150 will operate in the coaxial mode, yet allow forcooperation between the metal slug 150 and the outer conductor 135 toprovide a low-impedance capacitive bypass path therebetween at thefrequency of operation of the device 60.

Thus, in essence, the outer portion of the decoupling structure 120includes the inner conductor 125, the reduced diameter portion 138 ofthe outer conductor 135, the insulating sleeve 145, the ferrite member140 and the metal slug 150, all cooperating to form an RF rejectionfilter which operates like a lossy transmission line terminated by acapacitive reactance, the capacitive reactance providing a low-impedanceshunt path for the frequency of operation of the device 60 and the lossyline providing high attenuation of the harmonics of that frequency up tothe seventh harmonic. It is necessary for this purpose that thedecoupling structure 120 operate in the coaxial mode, whereby it isessential that the outer diameter of the metal slug 150 and the ferritemember 140 be maintained below a predetermined upper limit at which theywill begin to operate in a waveguide mode thereby propagating theharmonics of the frequency of operation. Further, the outer diameter ofthese filter elements must be maintained above a predetermined lowerlimit at which the metal slug 150 will cease to provide a capacitivecoupling to the outer conductor for the frequency of operation of thedevice 60. Thus, in order to achieve the desired results, it isnecessary that the outer diameters of the metal slug 150 and the ferritemember be small enough to insure coaxial mode operation to provide goodfilter response at RF frequencies including all harmonics up to theseventh harmonic, but large enough to provide a good capacitive loadingby the metal slug 150. Constructional models of the RF rejection filterformed by the ferrite member 140 and the metal slug according to thisinvention have produced attenuation values in excess of 50 per inchabove 5 GHz.

A lower coupling structure, generally designated by the numeral 160, isconnected at the lower end of the device 60. A tube 161 is disposedwithin the magnet yoke 78, coaxial therewith, and is connected at theupper end thereof to one end of the heater of the device 60 (not shown)and serves as an inner conductor of a coaxial transmission line, thelower end of the tube 161 carrying an internally threaded insert 162therein. The outer conductor of this coaxial transmission line isprovided by the magnet yoke 78. Disposed within and essentially liningthe magnet yoke 78 is a sleeve 163 of electrically insulating material,an inner conductor 164 being disposed against the inner surface ofsleeve 163' and telescopically overlapping a portion of the yoke 78 andhaving the outer end thereof closed by an end wall 165, the end wall 165having an opening therethrough for receiving the shank of a screw 166that engages in the complementarily threaded opening in the insert 162.The yoke 78, the tube 161, the insulating sleeve 163, the innerconductor 164 and the end wall 165 cooperate to provide a parallelresonant circuit including an inductive impedance and a capacitiveimpedance, the structure comprising a high impedance to RF energy toprevent propagation thereof from the terminal 166 to the power supplylead. More specifically, the distance between the lower adjacent end ofthe anode of the device 60 and the inner surface of the end wall 165 isequivalent to a quarter wavelength at the operating frequency of thedevice 60, and the yoke 78 and the inner conductor 164 telescopicallyoverlap a distance equivalent to one-eighth wavelength at the operatingfrequency of the device 60.

Disposed at the lower end of the device 60 is a conductive cap or cover,generally designated by the numeral 170, having a cylindrical sidewall171 disposed substantially concentric with the magnet yoke 78 andsurrounding the outer ends of the conductor 164 and the insulatingsleeve 163 and being closed at the lower end thereof by an end wall 172.Extending outwardly from the upper end of the cylindrical side wall 171substantially normal thereto is an attachment flange 173 attached to theouter surface'of the magnet flange 79 by suitable fastening means suchas screws 179. Extending outwardly from the sidewall 171 substantiallynormal thereto is a hollow cylindrical outer conductor 174 communicatingwith the interior of the cover through a complementarily shaped openingin the sidewall 171. A generally cylindrical inner conductor 175 isdisposed in the outer conductor 174 and extends into the interior of thecover 170, the inner conductor 175 having an externally threaded outerend and having a flattened inner end 176 having an opening therethroughfor receiving the terminal screw 166 whereby the inner conductor 175 issecurely fastened to the end wall 165 of the conductor 164. The innerconductor 175 is provided with a circular bearing flange 177 extendingradially outwardly therefrom intermediate the ends thereof. Disposedwithin the tube 174 is a cylindrical ferrite element 180 having anopening 181 extending axially therethrough for receiving therein theexternally threaded inner conductor 175 whereby the ferrite element 180is mounted on the inner conductor 175 in surrounding relationshiptherewith. Preferably, when assembled on the inner conductor 175, theinner end of the ferrite member 180 abuts against the bearing flange177. Also coupled to the outer end of the inner conductor 175 anddisposed within the tube 174 is a cylindrical metal slug having aninternally threaded opening extending axially therethrough for receivingtherein and threadedly engaging with the outer end of the externallythreaded outer conductor 175 whereby the metal slug 150 is securelymounted on the inner conductor 175 in surrounding relationshiptherewith. When assembled on the inner conductor 175, the inner end ofthe metal slug 190 abuts against the outer end of the ferrite member180, thereby holding the ferrite member 180 in the assembled positionagainst the bearing flange 177, Disposed within the tube 174 insurrounding relationship with the slug 190 and the ferrite member 180 isan insulating sleeve 185. A terminal screw 192 has the shank thereofreceived within the outer end of the internally threaded opening 191 inthe metal slug 190 providing a filament terminal for the device 60 forconnection of a lead 195 from the source of low-frequency AC operatingpotentials. The lead 195 may be held against the underside of the device60 by means of a clamp 196 (see FIG. 7) to prevent interference withother parts of the electronic oven apparatus. In operation, the innerconductor 175, the outer conductor 174, the ferrite member 180, themetal slug 190 and the insulating sleeve 185 cooperate to form an RFrejection filter for the frequency of operation of the device 60 and theharmonics thereof up to the seventh harmonic for preventing thepropagation of RF energy to the power supply circuitry over the lead195. This filament lead rejection filter is identical in operation tothe rejection filter formed by the conductors 125 and 135, the insulator126, the ferrite member 140 and the metal slug 150 in the couplingstructure 120 in the cathode lead of the device 60, whereby the detaileddescription of the operation of this filament lead filter will not nowbe repeated.

ln a constructional'example of an RF rejection filter constructed inaccordance with the present invention, the inner conductors 125 and 175were made of nickel-plated brass or aluminum alloy and wereapproximately 1.3 inches in length and had a diameter of approximately0.15 inch, the bearing flanges 129 and 177 having a diameter ofapproximately 0.38 inch; the outer conductor 135 was formed of copperand had an overall length of approximately 1.75 inches, the reduceddiameter section 138 having an inner diameter of approximately 0.463inch and the section 136 having an inner diameter of approximately 0.879inch; the outer conductor 174 has an inner diameter of approximately0.46 inch and is also formed of nickel-plated steel; the insulatingsleeves 145 and 185 are formed of a polytetrafluoroethylene resin suchas that sold under the trademark Teflon"; the ferrite members 140 and180 are formed of a suitable ferrite material and have a length ofapproximately 1 inch, an outer diameter of approximately 0.38 inch andan inner diameter of approximately 0.15 inch; the metal slugs 150 and190 were formed of an aluminum alloy and had a length of approximately0.75 inch, an outer diameter of approximately 0.438 inch and an innerdiameter of approximately 0.15 inch. 7

In the transmission line 90, the bullet 102, the tee 110, the innerconductor 106 and the tube 108 are shaped and arranged to provide aquarter wave transformer section at the frequency of operation of thedevice 60. More particularly, the shouldered portions of the bullet 102and the conductor 106, and the filter members in the transmission line90 have dimensions such that the impedance of the device 60 is matchedto the impedance of the transmission lines 80 and 90 that is in turnmatched to the impedance of the heating cavity 21. Likewise, the bullet102, the inner conductor 121, the outer conductor 124 and the innerconductor 130 are all shaped and arranged to provide a quarter wavetransformer section that assists in decoupling RF energy from the inputterminal 152 to prevent the propagation of RF energy into the powersupply. It is noted that the stepped configuration of the innerconductor 121 permits a shorter mechanical connection while maintainingan electrical characteristic equivalent to one-quarter wavelength of theoperating frequency of the device 60.

Because the potentials for operating the device 60 are derived from thevoltage doubler and rectifier circuit 95, neither the stud 101 formingthe inner conductor or any of the parts such as the yoke 76 and the tee110 forming the outer conductor of the coupling structure 100 can begrounded. However, it is highly desirable to ground the portion of thetransmission line 80 disposed to the rear of the rear baffle 40, and tothis end capacitive couplings have been provided between the rear lowertransmission line section 92 and the front lower transmission linesection 91; all as described in detail in the aforementioned copendingU.S application Ser. No. 788,214. Accordingly, the outer conductor ofthe rear transmission line section 92 can be grounded as on the casing11 and the baffle member 40, thereby to present only grounded parts toworkmen gaining access to the rear machinery compartment 45 through theremovable panel 15.

It is noted that the front panel 19 of the electronic heating apparatus10 is preferably removable. Due to the telescoping arrangement of thelower transmission line portions, the device 60 and the front portion 91of the transmission line 90 may be easily disengaged from the rearportion 92 of the transmission line 90 and removed from the lowermachinery compartment 35 through the front end thereof. Thus, theelectron discharge device is readily accessible from the front of therange for easy servicing.

1t further is pointed out that the tee 83, the entire rear transmissionline section 84, the tee and the rear lower transmission line section 92form a removable transmission line assembly that can be bodily movedrearwardly through the opening provided by the removable panel 15 formaintenance and repair of the parts. Such movement of the transmissionline assembly rearwardly is accomplished by simply loosening the clamps86, about the forward or inner legs of the tees 83 and 85 which freesthe tees 83 and 85 respectively from the transmission line sections 82and 92 and removing the screws holding the inner conductors of the linesections 82, 84 and 92 together. Due to the telescoping arrangement ofthe lower transmission line portions 91 and 92, there is no need toremove or disconnect any parts other than by the relative slidingmovement of the portions 91 and 92 with respect to each other.Reassembly of the parts is facilitated by flared ends on the outerconductor and the inner conductor of the transmission line section 91,respectively. It further is necessary to hold the removable transmissionline assembly in the assembled position, and to this end a spring 87under tension has been provided interconnecting the tee 85 and thebaffle wall 42, thus continually to urge the removable transmission lineassembly into the assembled operative position.

Another important feature of the transmission lines 80 and resides inthe fact that the tubes forming the inner and outer conductors thereofcan all be formed essentially of standard tubing shaped as required andcut to length, the tubing preferably being formed of copper, brass orother good electrically conductive metal. The tees 83, 85 and are alsoof standard configuration and are all identical one to the other, thetees preferably being formed of copper, brass or other material havinggood electrical conductivity. Finally, the insulators 126 and arepreferably all formed of a polytetrafluoroethylene resin such as thatsold under the trademark"Teflon".

From the above it will be seen that the liner 20 is effectively isolatedfrom the bottom machinery compartment 35 and the rear machinerycompartment 45 by the baffle members 30 and 40, respectively, thereby toprovide a more unifonn distribution of heat within the liner 20 and thusto permit good cooking therein. The entire generator 50, including thecrossed field discharge device 60 and the voltage doubler and rectifiercircuit 95 therefor, are housed within the bottom machinery compartment35 which provides a protecting housing therefor. The fan 66 serves tocool all of the electrical components of the generator 50 by drawing airinwardly through the screen 17 into the bottom machinery compartment 35and across the crossed field discharge device 60 and outwardly throughthe screen 18. The stream of air thus created is effectively preventedfrom coming into contact with the liner 20 due to the presence of thebaffle member 30 and 40. Also, the improved coupler structure andtransmission line 90 has been provided, the major portion of which canbe readily removed from the assembled relation with the liner and thedevice 60 for repair and service purposes through the removable panel15, and can thereafter be readily reassembled therewith.

Also, RF filters for filtering the frequency of operation of the device60 and the harmonics thereof up to the seventh harmonic have beenprovided in the coupling structure 120 between the device 60 and theEdison supply network and the voltage doubler and rectifier circuit 95to prevent propagation of RF energy thereto while not interfering withthe transmission of the DC and low-frequency AC operation potentials,therefrom to the device 60.

More particularly, there has been provided an improved RF rejectionfilter adaptable for use both in the cathode lead and the filament leadof a crossed field electron discharge device. Referring to the cathodelead application of the improved rejection filter, this filter includesthe inner conductor 125, the outer conductor 135, the ferrite member140, the metal slug 150 and the insulating sleeve 145, all cooperatingto provide high attenuation for the frequency of operation of the device60 and all harmonics thereof up to the seventh harmonic for preventingthe propagation of these frequencies to the voltage doubler andrectifier 95 and the source of low frequency AC operating potentials forthe device 60 over the lead 155. Referring to the filament leadapplication, this improved rejection filter includes the inner conductor175, the outer conductor 174, the ferrite member 180, and the metal slug190, and the insulating sleeve 185 all cooperating to provide highattenuation for the frequency of operation of the device 60 and allharmonics thereof up to the seventh harmonic to prevent the propagationof these frequencies to the source of low low-frequency potential forthe device 60 over the lead 195.

ln addition there has been provided an improved RF rejection filter ofsimple and economical construction and specially adapted for use withthe microwave generator of an electronic oven for providing highattenuation over a broad band of RF frequencies.

While there has been described what is at present con sidered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modification as fall within the truespirit and scope of the invention.

What is claimed is:

1. ln an electronic heating apparatus adapted for use with a source ofDC operating potential having at least one terminal and with a source oflow-frequency AC energy having at lease one terminal, structure defininga cooking cavity, a generator for generating electromagnetic wave energyof a predetermined ultrahigh frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity, a hollow outer conductor coupled to one output terminal of saidgenerator and extending outwardly therefrom and terminating in an outerend, an inner conductor disposed in said outer conductor and coupled tothe other output terminal of said generator and extending outwardlytherefrom and terminating in an outer end adjacent to the outer end ofsaid outer conductor, an insulating sleeve disposed within said outerconductor in surrounding relationship with said inner conductor, meanscoupling the DC source terminal and the low-frequency AC source terminalto the outer end of said inner conductor, a ferrite filter elementdisposed in said inner conductor intermediate the outer end thereof andsaid generator output terminals and being constructed and arranged tooperate as a section of lossy transmission line in the coaxial mode forproviding high-transmission losses at selected harmonics of thepredetermined frequency, and a conductive filter element disposed insaid inner conductor in end-to-end relationship and longitudinalalignment with said ferrite filter element and connected thereto andbeing constructed and arranged to cooperate with said insulating sleeveand said outer conductor for providing a low-impedance bypass pathbetween said inner and outer conductors at the predetermined frequency,whereby propagation of ultrahigh frequency energy at the predeterminedfrequency and selected harmonics thereof from the generator to the outerends of said conductors is highly attenuated without interfering withthe transmission of the DC operating potentials and the low frequency ACenergy from the sources thereof to the generator.

2. The electronic heating apparatus set forth in claim 1, wherein saidinner and outer conductors and said ferrite filter element are eachcylindrical in shape and circular in cross section, the outer diameterof said ferrite filter element being such as to provide high attenuationof all of the harmonics of the predetermined frequency up to the seventhharmonic.

3. The electronic heating apparatus set forth in claim 1, wherein saidinner and outer conductors and said conductive filter element are eachcylindrical in shape and circular in cross section, the outer diameterof said conductive filter element being such as to provide a capacitivebypass path between said inner and outer conductors at the predeterminedfrequency.

4. The electronic heating apparatus set forth in claim 1, wherein saidinner and outer conductors and said ferrite filter element and saidconductive filter element are all cylindrical in shape and circular incross section, the outer diameters of said ferrite filter element andsaid conductive filter element being substantially equal to each other.

5. The electronic heating apparatus set forth in claim 1, wherein saidouter conductor overlaps said conductive filter element a distance notgreater than one-quarter of the wavelength of the predeterminedfrequency.

6. The electronic heating apparatus set forth in claim 1, wherein saidinner conductor comprises an externally threaded rod, said ferritefilter element and said conductive filter element each being cylindricalin shape and having an internally threaded opening extending axiallytherethrough, said inner conductor being threadedly engaged with theinternally threaded openings through said ferrite filter element andsaid conductive filter element for coupling said filter elements to saidinner conductor.

7. In an electronic heating apparatus adapted for use with a source ofDC operating potential having at least one terminal and with a source oflow-frequency AC energy having a pair of terminals, structure defining acooking cavity, a generator for generating electromagnetic wave energyof a predetermined ultrahigh frequency and having a pair of outputterminals and a low-frequency AC input terminal, a transmission linecoupling said generator to said cooking cavity, a first hollow outerconductor coupled to one output terminal of said generator and extendingoutwardly therefrom and terminating in an outer end, a first innerconductor disposed in said outer conductor and coupled to the otheroutput terminal of said generator and extending outwardly therefrom andterminating in an outer end adjacent to the outer end of said outerconductor, a first insulating sleeve disposed within said outerconductor in surrounding relationship with said inner conductor, meanscoupling the DC source terminal and the low-frequency AC source terminalto the outer end of said inner conductor, a first ferrite filter elementdisposed in said inner conductor intermediate the outer end thereof andsaid generator output terminals and being constructed and arranged tooperate as a section of lossy transmission line for providinghigh-transmission losses at selected harmonics of the predeterminedfrequency, a first conductive filter element disposed in said innerconductor in longitudinal alignment with said ferrite filter element andconnected thereto and being constructed and arranged to cooperate withsaid insulating sleeve and said outer conductor for providing alow-impedance bypass path between said inner and outer conductors at thepredetermined frequency, a second hollow outer conductor coupled to saidone output terminal of said generator and disposed adjacent to saidlow-frequency AC input terminal and extending outwardly therefrom andterminating in an outer end, a second inner conductor disposed in saidsecond outer conductor and coupled to said low-frequency AC inputterminal of said generator and extending outwardly therefrom andterminating in an outer end adjacent to the outer end of said outerconductor, a second insulating sleeve disposed within said second outerconductor in surrounding relationship with said second inner conductor,means coupling the other of said lowfrequency AC source terminals to theouter end of said second inner conductor, a second ferrite filterelement disposed in said second inner conductor intermediate the outerend thereof and said generator low-frequency AC input terminal and beingconstructed and arranged to operate as a section of lossy transmissionline for providing high-transmission losses at selected harmonics of thepredetermined frequency and a second conductive filter element disposedin said second inner conductor in longitudinal alignment with saidsecond ferrite filter element and connected thereto and beingconstructed and arranged to cooperate with said second insulating sleeveand said second outer conductor for providing a low-impedance bypasspath between said second inner and outer conductors for thepredetermined frequency, whereby propagation of ultrahigh frequency atthe predetermined frequency and harmonics thereof from the generator tothe outer ends of said first and second conductors is highly attenuatedwithout interfering with the transmission of the DC operating potentialsand the low-frequency AC energy from the sources thereof to thegenerator.

8. The electronic heating apparatus set forth in claim 7, wherein saidfirst ferrite filter element is identical in construction to said secondferrite filter element, and wherein said first conductive filter elementis identical in construction to said second conductive filter element.

1. In an electronic heating apparatus adapted for use with a source ofDC operating potentiAl having at least one terminal and with a source oflow-frequency AC energy having at least one terminal, structure defininga cooking cavity, a generator for generating electromagnetic wave energyof a predetermined ultrahigh frequency and having a pair of outputterminals, a transmission line coupling said generator to said cookingcavity, a hollow outer conductor coupled to one output terminal of saidgenerator and extending outwardly therefrom and terminating in an outerend, an inner conductor disposed in said outer conductor and coupled tothe other output terminal of said generator and extending outwardlytherefrom and terminating in an outer end adjacent to the outer end ofsaid outer conductor, an insulating sleeve disposed within said outerconductor in surrounding relationship with said inner conductor, meanscoupling the DC source terminal and the low-frequency AC source terminalto the outer end of said inner conductor, a ferrite filter elementdisposed in said inner conductor intermediate the outer end thereof andsaid generator output terminals and being constructed and arranged tooperate as a section of lossy transmission line in the coaxial mode forproviding high-transmission losses at selected harmonics of thepredetermined frequency, and a conductive filter element disposed insaid inner conductor in end-to-end relationship and longitudinalalignment with said ferrite filter element and connected thereto andbeing constructed and arranged to cooperate with said insulating sleeveand said outer conductor for providing a low-impedance bypass pathbetween said inner and outer conductors at the predetermined frequency,whereby propagation of ultrahigh frequency energy at the predeterminedfrequency and selected harmonics thereof from the generator to the outerends of said conductors is highly attenuated without interfering withthe transmission of the DC operating potentials and the low frequency ACenergy from the sources thereof to the generator.
 2. The electronicheating apparatus set forth in claim 1, wherein said inner and outerconductors and said ferrite filter element are each cylindrical in shapeand circular in cross section, the outer diameter of said ferrite filterelement being such as to provide high attenuation of all of theharmonics of the predetermined frequency up to the seventh harmonic. 3.The electronic heating apparatus set forth in claim 1, wherein saidinner and outer conductors and said conductive filter element are eachcylindrical in shape and circular in cross section, the outer diameterof said conductive filter element being such as to provide a capacitivebypass path between said inner and outer conductors at the predeterminedfrequency.
 4. The electronic heating apparatus set forth in claim 1,wherein said inner and outer conductors and said ferrite filter elementand said conductive filter element are all cylindrical in shape andcircular in cross section, the outer diameters of said ferrite filterelement and said conductive filter element being substantially equal toeach other.
 5. The electronic heating apparatus set forth in claim 1,wherein said outer conductor overlaps said conductive filter element adistance not greater than one-quarter of the wavelength of thepredetermined frequency.
 6. The electronic heating apparatus set forthin claim 1, wherein said inner conductor comprises an externallythreaded rod, said ferrite filter element and said conductive filterelement each being cylindrical in shape and having an internallythreaded opening extending axially therethrough, said inner conductorbeing threadedly engaged with the internally threaded openings throughsaid ferrite filter element and said conductive filter element forcoupling said filter elements to said inner conductor.
 7. In anelectronic heating apparatus adapted for use with a source of DCoperating potential having at least one terminal and with a source oflow-frequency AC energy having a pair of terminals, structure defiNing acooking cavity, a generator for generating electromagnetic wave energyof a predetermined ultrahigh frequency and having a pair of outputterminals and a low-frequency AC input terminal, a transmission linecoupling said generator to said cooking cavity, a first hollow outerconductor coupled to one output terminal of said generator and extendingoutwardly therefrom and terminating in an outer end, a first innerconductor disposed in said outer conductor and coupled to the otheroutput terminal of said generator and extending outwardly therefrom andterminating in an outer end adjacent to the outer end of said outerconductor, a first insulating sleeve disposed within said outerconductor in surrounding relationship with said inner conductor, meanscoupling the DC source terminal and the low-frequency AC source terminalto the outer end of said inner conductor, a first ferrite filter elementdisposed in said inner conductor intermediate the outer end thereof andsaid generator output terminals and being constructed and arranged tooperate as a section of lossy transmission line for providinghigh-transmission losses at selected harmonics of the predeterminedfrequency, a first conductive filter element disposed in said innerconductor in longitudinal alignment with said ferrite filter element andconnected thereto and being constructed and arranged to cooperate withsaid insulating sleeve and said outer conductor for providing alow-impedance bypass path between said inner and outer conductors at thepredetermined frequency, a second hollow outer conductor coupled to saidone output terminal of said generator and disposed adjacent to saidlow-frequency AC input terminal and extending outwardly therefrom andterminating in an outer end, a second inner conductor disposed in saidsecond outer conductor and coupled to said low-frequency AC inputterminal of said generator and extending outwardly therefrom andterminating in an outer end adjacent to the outer end of said outerconductor, a second insulating sleeve disposed within said second outerconductor in surrounding relationship with said second inner conductor,means coupling the other of said low-frequency AC source terminals tothe outer end of said second inner conductor, a second ferrite filterelement disposed in said second inner conductor intermediate the outerend thereof and said generator low-frequency AC input terminal and beingconstructed and arranged to operate as a section of lossy transmissionline for providing high-transmission losses at selected harmonics of thepredetermined frequency, and a second conductive filter element disposedin said second inner conductor in longitudinal alignment with saidsecond ferrite filter element and connected thereto and beingconstructed and arranged to cooperate with said second insulating sleeveand said second outer conductor for providing a low-impedance bypasspath between said second inner and outer conductors for thepredetermined frequency, whereby propagation of ultrahigh frequencyenergy at the predetermined frequency and harmonics thereof from thegenerator to the outer ends of said first and second conductors ishighly attenuated without interfering with the transmission of the DCoperating potentials and the low-frequency AC energy from the sourcesthereof to the generator.
 8. The electronic heating apparatus set forthin claim 7, wherein said first ferrite filter element is identical inconstruction to said second ferrite filter element, and wherein saidfirst conductive filter element is identical in construction to saidsecond conductive filter element.